This invention relates to a method for preparing aqueous dispersions of organic polymers, and more particularly, to an emulsion polymerization method carried out in the presence of a stabilizing emulsifier and a polymer of a sulfo ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid. A main embodiment of this invention relates to the preparation of microspheres having liquid centers and seamless rigid walls of an organic polymer.
Microspheres encompassing various kinds of liquids are quite useful in the plastic industry. For example, hollow thermoplastic microspheres are utilized as lightweight fillers for paints and paper coatings. Also porous foams are often prepared by heating thermoplastic microspheres having volatile liquid centers. These porous foams have many useful properties such as light weight, high insulating character and buoyancy. Such foams have been used in diverse fields such as packaging, refrigeration, flotation equipment, construction and the like. Various liquids are also encapsulated for ease of handling and the like, and later released from the capsule by diffusion, pressure rupture, heat, etc. Examples are insecticides, pharmaceuticals, inks, fire retardants and the like.
Conventionally microspheres having liquid centers are prepared by suspension polymerization processes. In such processes liquids are usually encapsulated in the microspheres by dispersing the liquid to be encapsulated in the monomer phase before polymerization. Known suspension polymerization procedures for encapsulating liquids comprise the steps of (1) suspending the monomeric components and the liquid to be encapsulated in water with the aid of a suspension stabilizer such as colloidal silica and (2) polymerizing the monomeric components by adding an oil-soluble free radical type catalyst and a small amount of heat. Unfortunately, suspension polymerization proceeds at a relatively slow rate. Also microspheres having diameters less than 2 microns which are often desired in certain coating compositions are not readily prepared by suspension polymerization. While solid polymer particles having diameters less than 2 microns are easily prepared by emulsion polymerization, microspheres having liquid centers cannot be satisfactorily prepared by ordinary emulsion polymerization methods. The difficulty in preparing microspheres by conventional emulsion polymerization is due to the nature of emulsion polymerization which involves the migration of unreacted monomer from the oil phase through the aqueous phase to a micelle where the monomer is then polymerized. As a result the liquid to be encapsulated appears as a separate oil phase.
In view of the potential utility of polymer particles having a uniform size, particularly small microspheres having liquid or hollow centers, and the lack of a suitable method for preparing them, it would be highly desirable to provide such a suitable method.